Problem: A jar contains $3$ red balls, $9$ green balls, and $6$ blue balls. If a ball is randomly chosen, what is the probability that it is not blue?
There are $3 + 9 + 6 = 18$ balls in the jar. There are $6$ blue balls. That means $18 - 6 = 12$ are not blue. The probability is $ \frac{12}{18} = \dfrac{2}{3}$.